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From: "Guy Harris" <guy@xxxxxxxxxxxx>
Sent: Friday, August 20, 2010 10:48 AM
To: "Community support list for Wireshark" <wireshark-users@xxxxxxxxxxxxx>
Subject: Re: [Wireshark-users] Wireshark time behind the actual time
On Aug 20, 2010, at 4:09 AM, Gary Chaulklin wrote:
I am working with an employee in a remote location. I am getting him to
run FiddlerCap and Wireshark to get plain text and packet level traces to
troubleshoot an issue.
The FiddlerCap trace matched the users experience in terms of watching
the clock on the PC, but Wireshark is about 20 seconds behind the actual
time. As the trace goes on the time of the Wireshark packets gets more
behind the actual time so that by the end of a 5 minute trace it is over
60 seconds behind. The FiddlerCap trace which records timings to the
millisecond always seems to be correct.
Any ideas as to why the Wireshark time would be behind the actual time
for this remote user? I have worked with dozens of users over a period
of several years with first Ethereal then Wireshark and have never seen
this particular issue.
The remote user and I run the same Windows XP Professional PCs.
...which means the capture is being done using WinPcap, and thus the time
stamps are coming from WinPcap.
As I remember, WinPcap has multiple time stamping modes:
In one mode, it queries the system time stamp; in that mode, the time
stamps will obviously match the time stamp on the clock on the PC (whether
the PC's clock is the "actual time" is another matter), but, at least
according to
http://www.osronline.com/ddkx/kmarch/k105_41iq.htm
"System time is typically updated approximately every ten milliseconds.",
which means that the time stamp resolution is only 10ms or so.
In at least some of the other modes, it uses the performance counter; in
that mode, you can get higher-resolution time stamps, but the time can
drift from the system time.
I'll let the WinPcap developers give more details and corrections to the
above.
There is not much to add to it. WinPcap by default uses a timestamping
source that is quite accurate but gets synchronized with the system clock
only at the beginning of a capture (it's actually more complicated than
this. The point is that it doesn't resync during the capture). There is an
option (through the registry) to change the timestamping mode and use the
system time. The problem with that is that the system time gets updated
every X milliseconds (where X can be something between 1 and 15 or so).
Have a nice day
GV
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